It's probably for the same reason that anyone can download the times recipe, even without a paid subscription - the times 'firewall' is very thin - it's just a bit of javascript that pops up if the web address contains a particular code at the end. To the point where, if you're reading on the web and you get a "you've accessed your alloted free articles this month message", all you have to do is delete the extra code at the end of the web address and you can access the article you're trying to read.

calibre, just because of how it works in the first place, ignores the javascript completely. It wasn't even designed to 'beat' the times system, it just happens to work in a way that beats it unintentionally.